Positive Current Cross-Correlations In A Highly Transparent Normal-Superconducting Beam Splitter Due To Synchronized Andreev And Inverse Andreev Reflections

Predictions are established for linear differential current-current cross-correlations dS(a,b)/dV in a symmetrically biased three-terminal normal metal-superconductor-normal metal (NSN) device. Highly transparent contacts turn out to be especially interesting because they feature positive dS(a,b)/dV. At high transparency, processes based on crossed Andreev reflection (CAR) contribute only negligibly to the current and to dS(a,b)/dV. Under these circumstances, current-current cross-correlations can be plausibly interpreted as a coherent coupling between the two NS interfaces in the form of synchronized Andreev and inverse Andreev reflections (AR-(AR) over bar), corresponding to the process where a pair of electronlike quasiparticles and a pair of holelike quasiparticles arrive from the normal electrodes and annihilate in the superconductor. Hence, positive dS(a,b)/dV does not automatically imply CAR. For tunnel contacts, dS(a,b)/dV is positive because of CAR. In between these two extremities, at intermediate transparencies, dS(a,b)/dV is negative because both processes which cause positive correlations occur only with small amplitude. We use scattering theory to obtain analytic expressions for current and noise and microscopic calculation using a tight-binding model in order to obtain a clear interpretation of the physical processes.